The present invention relates to a snap-action fastening device for control and/or signalling units provided with snap-action locking means disengageable through the action of a tool, such as a screwdriver. Devices mountable on a switchboard and containing push-buttons and control levers, as well as signalling means, such as lamps, has been well known for a long time. Among such known devices are and largely used those disclosed and claimed in the European Patents No. 0,105,387 and 0,199,344 granted to the applicant of the present application, to which reference is made. These devices are of the kind having fastening means located at the rear of the switchboard and comprise a first external member protruding from a switchboard into which is bayonett inserted, a second member provided with a fastening ring nut to fasten the device to the switchboard to prevent unscrewing of the same by operating from the external side of the switchboard. The second member, located at the rear side of the board can receive a plugged flange at which may be mounted control and/or signalling units which, according to the European Patent No. 0,105,387, are fastened by means of screws engageable in threaded holes in the flange and, according to the European Patent No. 0,199,344 are axially inserted into protruding fingers of the flange itself.
The above mentioned devices operate rather well, but are suited only for large size and high price push-buttons mountable only in relatively large and high cost control boards where both the large size and the high cost can be justified.
However, the problem of providing smaller size and less expensive devices to be used in smaller and less expensive control boards and being fastenable at the rear side of a switchboard and capable of bearing control and/or signalling units easily mountable thereon was not resolved.